This is a continuation-in-part application of our copending application Ser. No. 561,149 filed on Dec. 14, 1983, now U.S. Pat. No. 4,515,203, which is a continuation of application Ser. No. 250,041 filed on Apr. 1, 1981, now abandoned.
This invention relates to a method for producing low carbon killed steels, especially billets having a small size in cross-section, by a continuous casting process using an electromagnetic stirring technique.
In a continuous steel casting, there arise problems of defects as detected by an ultrasonic test, e.g., inclusions occurring in a sub-surface or internal portion of a continuously cast strand (hereinafter referred to as "the c.c. strand" for brevity) in its solidifying stage and shrinkage cavities are produced in axial center portions of the c.c. strand. In addition, strong segregations occur in the c.c. strands at a high temperature in continuous casting operations.
Various attempts have thus far been made to eliminate the internal defects of the c.c. strands, including center segregations and shrinkage cavities, through a single electromagnetic stirring either within a mold or in a secondary cooling zone, and severing tip ends of growing crystals with fluidic movements of molten steel to produce a large quantity of equiaxed crystal nuclei, thereby expanding the equiaxed crystal zone in the center portion of the c.c. strands. However, none of the attempts have succeeded in sufficiently reducing the ratio of center segregations and irregularities of center segregations in the axial direction of the c.c. strands, failing to produce steel castings of satisfactory quality.
On the other hand, the nozzle diameter (about 12-15 mm) of a continuous type billet-casting machine is inherently smaller than that of a continuous type bloom-casting machine due to its structural features, so that when molten steel containing Al in a high concentration is treated by a billet-casting machine, Al.sub.2 O.sub.3 -inclusions are attached to the nozzle resulting in a nozzle blockage which makes the billet casting difficult. For this reason, Si-killed steels deoxidized with Si are generally applied to such a billet-casting machine, but a number of blow holes exist on the resulting billet due to insufficient deoxidization therein, thereby causing scars and/or cracks on the surface of the product obtained from the billet by rolling and/or forging work.
Moreover, where a billet-casting machine is used, the molten steel flows at a high speed within a narrow space in the mold thereby drawing inclusions into the lower region of the molten steel so that the inclusions cannot be eliminated from the molten steel, resulting in a large amount of inclusions in the billet, producing a product which is inferior in quality when compared with blooms having a large size in cross-section.
In the production of low carbon killed steels by a billet-casting machine, center cavities are formed as a peculiarity of the steels. When the resulting c.c. strand was subjected to a succeeding heat treatment, scales occur not only on the surface of the c.c. strand, but also within the cavities open to the outside, and the scales within the cavities remain as inclusions in rolling work and/or cause cracks in cold forging work.
In order to obtain quality low carbon killed steel billets, therefore, blooms having a large size in cross-section have been produced by a continuous type bloom-casting machine, first, and then subjected to a rolling process. This process requires additional heating and rolling processes, causing an increase in the production cost.